In recent years there has been an increasing awareness of the prevalence of lung function abnormalities, resulting in mass testing programs, industrial health surveillance programs, and federal disability evaluations, as well as initial detection of differential diagnosis in a variety of medical environments.
U.S. Pat. Nos. 3,086,515 and 3,589,190 disclose bellows-type pulmonary function testing equipment; other types include manometer systems in which an inverted cannister is supported for vertical movement while being partially emmersed in a body of liquid, and electronic systems in which volume and flow rates are electronically measured. All such systems, which are commonly utilized for measuring Forced Vital Capacity (FVC) and changing flow rates represented by Forced Expiratory Volumes (FEV), are commonly referred to as "spirometers".
Although the spirometers disclosed in the afore-mentioned patents, as well as other known spirometer constructions, have been proven for their reliability and accuracy, there have been various uncertainties relating, for example, to possible leakage of the volume collecting devices, or to possible variations in chart timer speed from one instrument to the next. Also, some of the newer electronic spirometers have been proclaimed as advantageous but there has been little available to test their validity or correlate their values to the classic prediction normal standards.
The basic pulmonary function test requires a subject to maximally expire air and then forcefully exhale all of his distendible lung volume as quickly (to assess flow obstruction) and completely (to assess volumetric restrictions) as possible. As it is the repeatability of results that serves as the guide to prove validity, it is also important that the testing device is itself repeatable in simultaneously deriving flow and volume values.
Applicant has previously developed and disclosed the use of a large cylinder with a piston having a geared rack driven by a one-third horsepower geared synchronous motor for the purpose of testing the performance of spirometers. Such an apparatus is designed to discharge a precise volume of gas at uniform flow rates and is both bulky and expensive, characteristics which render such calibrating equipment unsuitable for use in the periodic (preferably daily) checking of spirometers in field use. Although the need for an inexpensive, compact, and easily operated device for calibrating spirometers has been recognized for a number of years, devices meeting those requirements have not heretofore been known or become available.